Few injuries have as much negative impact on a runner’s training program than a stress fracture . The treatment involves the cessation of running for six to eight weeks (12-15 weeks in severe cases) to allow the bone to heal. Stress fractures affect elite and recreational runners alike. Women’s marathon record holder Paula Radcliffe has suffered several stress fractures throughout her career. I know runners whose marathon times range from 3:20 to 4:40 who have all suffered stress fractures .
Research studies shed light on the biomechanical factors that increase your predisposition to stress fractures , the differences between female and male runners that suffer stress fractures and a simple exercise.
What is a stress fracture ?
A stress fracture is a partial or incomplete bone fracture caused by repetitive stress. This is in contrast to most bone fractures which are caused by a single severe trauma. Two of the more common stress fractures sites for runners are the tibia and metatarsals.
What factors predispose a runner to stress fractures ?
It was commonly thought that bone density was a primary risk factor. The weaker (less dense) the bone , the greater the likelihood of the runner suffering a stress fracture . However, a 2005 German study linked calf muscle fatigue with the increased incidence of metatarsal stress fracture . The calf muscles absorb force during footstrike thereby limiting the amount of force traveling through the ankle joint. In addition, calf muscles control the movement of the ankle joint. As calf muscles fatigue, both their shock absorption and motion control capability decline, resulting in excessive force and pronation at the ankle joint and foot, thereby increasing metatarsal stress, leading to stress fracture .
Calf muscles play an important role in the risk reduction of tibial stress fractures too. A recent study of women runners conducted at the University of Minnesota concluded that the difference between those women who suffered tibial stress fractures and those who didn’t was the size of their calf muscles. Those with larger (i.e. stronger) calf muscles were far less prone to stress fractures .
Dr. Moira Petit, an Associate Professor of Kinesiology at the University of Minnesota and an author of the study said (as quoted in the NY Times): “It does seem as if strengthening the calf muscles may be a very easy way to reduce fracture risk”.
These two studies dovetail nicely, concluding that:
Strengthening your calf muscles and increasing their resistance to fatigue will decrease the risk of both metatarsal and tibial stress fractures
Interestingly, in other research conducted by Dr. Petit, the same correlation between calf muscle size and the incidence tibial stress fractures does not hold true for male runners.
Another research study at the University of Iowa showed that reducing stride length by 10% reduced the stress on the tibia sufficiently to lower the incidence of stress fracture . Brent Edwards, the lead author of the study concluded: “If you spend less time in the flight phase of running” – meaning in the air – “you’ll hit the ground with less force.” Although a reduction in stride length means hitting the ground more often over a given distance, this is outweighed by the benefits of lower force upon footstrike.
How to implement these results to reduce your risk of a stress fracture ?
The results of these studies are good news for runners. Decreasing your risk of suffering a stress fracture is well within your control. Of course, other factors such as genetics, training, hormone levels and diet may also affect the stress fracture risk. But strengthening a muscle is very easy to address. Achieving enough of an increase in muscle strength (10% – 15%) to decrease stress fracture risk can easily be accomplished in a 4-6 week time period. Similarly, consciously shortening your stride is straightforward.
To strengthen your calf muscles and increase their fatigue resistance, perform one-legged standing calf raises. One-legged is more effective than two-legged raises, where the stronger leg will compensate for the weaker one. Equally as important, one-legged raises mimic running, when only one leg is in contact with the ground. Slowly rise up on your toes and sink back slowly so that your heel is below the stair but not at the extreme end of its range of motion. Start with one set of 5 repetitions, gradually working up to 3 sets of 12-15 reps, 3 times per week.
To decrease your stride length by 100%, Mr. Edwards suggests: “10% is about as much as you can shorten your stride without it feeling uncomfortable”. Visiting a biomechanics lab is unnecessary!